selftests/mm: migration: make nthreads represent number of working threads

selftests/mm: migration: make nthreads represent number of working threads

Fixture setup sets self->nthreads to number of available CPUs minus 1 and
then each test creates 'self->nthreads - 1' threads or processes, so
essentially nthreads counts the worker tasks and the main task.

Make nthreads represent the number of spawned tasks to simplify
thread/process creation and teardown.

While on it, make the fixture setup skip the tests if there are not enough
CPUs or NUMA nodes instead of checking this in each test.

Link: https://lore.kernel.org/20260511162840.375890-4-rppt@kernel.org
Reviewed-by: Luiz Capitulino <luizcap@redhat.com>
Tested-by: Luiz Capitulino <luizcap@redhat.com>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Donet Tom <donettom@linux.ibm.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Liam Howlett <liam@infradead.org>
Cc: Li Wang <li.wang@linux.dev>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Sarthak Sharma <sarthak.sharma@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>